Driving systems with asynchronous motors and adjustable speed by variation of the supplying voltage frequency are known. These systems have the disadvantage that the speed varying device is expensive.
It is also know to provide driving systems which employ direct-current motors which have collectors and brushes and which are driven with a high operating frequency, but which have the disadvantage that they are less reliable.
Driving systems with asynchronous motors and speed variation brought about through couplings are not able to provide rapid dynamic behavior.